[llvm-dev] Should rint and nearbyint be always constrained?

Tue Mar 3 04:31:51 PST 2020

Hi Andy,

On Mon, 2 Mar 2020 at 23:59, Kaylor, Andrew via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Some clarification after getting feedback from Craig Topper….
>
>
>
> It’s probably best to say in the documentation that the llvm.nearbyint and
> llvm.rint functions “assume the default rounding mode, roundToNearest”.
> This will allow the optimizer to transform them as if they were rounding to
> nearest without requiring backends to use an encoding that enforces
> roundToNearest as the rounding mode for these operations. On modern x86
> targets we can encode it either way, but it seems more consistent to
> continue using the current encoding which tells the processor to use the
> current rounding mode. For other targets (including cases where x86 is
> forced to use x87 instructions), it may be much easier to leave this at the
> discretion of the backend.
>
>
>
> Also, we should take care to document the non-constrained forms of these
> intrinsics in a way that makes clear that we are “assuming” and not
> requiring that the operation has no side effects.
>

Note that these aspects are shared by most other FP operations and already
discussed in the LangRef section <
https://llvm.org/docs/LangRef.html#floating-point-environment> which
currently reads:

> The default LLVM floating-point environment assumes that floating-point
instructions do not have side effects. Results assume the round-to-nearest
rounding mode. No floating-point exception state is maintained in this
environment. Therefore, there is no attempt to create or preserve invalid
operation (SNaN) or division-by-zero exceptions.
>
>  The benefit of this exception-free assumption is that floating-point
operations may be speculated freely without any other fast-math relaxations
to the floating-point model.
>
> Code that requires different behavior than this should use the
Constrained Floating-Point Intrinsics.

Your explanation of the implications for optimizers and backends seems like
a useful addition to this section. As many intrinsics (not just
nearbyint/rint) and instructions (fadd, fmul, etc.) behave this way, I
think it would be more useful to consolidate all the information into this
section and reference it from the relevant "Semantics" sections.

While we're on it, let me point out the consequences of breaking these
assumptions are still fuzzy even with your clarifications. In general, when
a compiler "assumes" something that is not actually true, it's useful to
specify what exactly happens when the assumption is actually false, e.g.
the result is an undefined value (undef/poison), or a non-deterministic
choice is made (e.g. branching on poison, at the moment), or Undefined
Behavior happens. In this sense, I wonder what should happen when the
assumptions about rounding mode and FP exception state are broken? If it's
going to take broader discussion to agree on an answer, that's probably out
of scope for this thread, but perhaps there's a clear answer that just
wasn't written down so far?

For the constrained version of nearbyint, we will require that the inexact
> exception is not raised (to be consistent with iEEE 754-2019’s
> roundToIntegral operations) and for the constrained version of rint we will
> require that the inexact exception is raised (to be consistent with iEEE
> 754-2019’s roundToIntegralExact operation), but for the non-constrained
> forms it should be clear that the backend is free to implement this in the
> most efficient way possible, without regard to FP exception behavior.
>
>
>
> Finally, I see now the problem with documenting these in terms of the IEEE
> operations, given that IEEE 754-2019 doesn’t describe an operation that
> uses the current rounding mode without knowing what that is. I see this as
> a problem of documentation rather than one that presents any difficulty for
> the implementation.
>

I'm not quite sure what you mean by "uses the current rounding without
knowing what it is" --are you referring to the wobbly uncertainty caused by
optimizations assuming one rounding mode but runtime code possibly using a
different dynamic rounding mode? If so, explicitly defining what happens
when dynamic and "assumed" rounding mode don't match (see above) also
addresses this problem. Then the operations can be described like this:

> If a rounding mode is assumed [RNE for non-constrained intrinsic or
roundingMode argument != fpround.dynamic] and the current dynamic rounding
mode differs from the assumed rounding mode, [pick one: behavior is
undefined / result is poison / ...]. Otherwise, X operation is performed
with the current dynamic rounding mode [which equals the statically assumed
rounding mode if this clause applies].

Best regards,
Hanna

> Here are some suggested wordings for the “Semantics” section of the
> langref for these functions:
>
>
>
> llvm.nearbyint::semantics
>
>
>
> This function returns the same value as one of the IEEE 754-2019
> roundToIntegral operations using the current rounding mode. The optimizer
> may assume that actual rounding mode is roundToNearest (IEEE 754:
> roundTiesToEven), but backends may encode this operation either using that
> rounding mode explicitly or using the dynamic rounding mode from the
> floating point environment. The optimizer may assume that the operation has
> no side effects and raises no FP exceptions, but backends may encode this
> operation using either instructions that raise exceptions or instructions
> that do not. The FP exceptions are assumed to be ignored.
>
>
>
> llvm.rint (delete, or identical semantics to llvm.nearbyint)
>
>
>
> llvm.experimental.constrained.nearbyint::semantics
>
>
>
> This function returns the same value as one of the IEEE 754-2019
> roundToIntegral operations. If the roundingMode argument is
> fpround.dynamic, the behavior corresponds to whichever of the
> roundToIntegral operations matches the dynamic rounding mode when the
> operation is executed. The optimizer may not assume any rounding mode in
> this case, and backends must encode the operation in a way that uses the
> dynamic rounding mode. Otherwise, the rounding mode may be assumed to be
> that described by the roundingMode argument and backends may either use
> instructions that encode that rounding mode explicitly or use the current
> rounding mode from the FP environment.
>
>
>
> The optimizer may assume that this operation does not raise the inexact
> exception when the return value differs from the input value, and if the
> exceptionBehavior argument is not fpexcept.ignore, the backend must encode
> this operation using instructions that guarantee that the inexact exception
> is not raised. If the exceptionBehavior argument is fpexcept.ignore,
> backends may encode this operation using either instructions that raise
> exceptions or instructions that do not.
>
>
>
> llvm.experimental.constrained.rint::semantics
>
>
>
> This function returns the same value as the IEEE 754-2019
> roundToIntegralExact operation. If the roundingMode argument is
> fpround.dynamic, the behavior uses to the dynamic rounding mode when the
> operation is executed. The optimizer may not assume any rounding mode in
> this case, and backends must encode the operation in a way that uses the
> dynamic rounding mode. Otherwise, the rounding mode may be assumed to be
> that described by the roundingMode argument and backends may either use
> instructions that encode that rounding mode explicitly or use the current
> rounding mode from the FP environment.
>
> If the exceptionBehavior argument is not fpexcept.ignore, the optimizer
> must assume that this operation will raise the inexact exception when the
> return value differs from the input value and the backend must encode this
> operation using instructions that guarantee that the inexact exception is
> raised in that case. If the exceptionBehavior argument is fpexcept.ignore,
> backends may encode this operation using either instructions that raise
> exceptions or instructions that do not.
>
>
>
>
>
> I’d like to also say that these intrinsics can be lowered to the
> corresponding libm functions, but I’m not sure all libm implementations
> meet the requirements above.
>
>
>
> -Andy
>
>
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *Kaylor,
> Andrew via llvm-dev
> *Sent:* Monday, March 02, 2020 9:56 AM
> *To:* Serge Pavlov <sepavloff at gmail.com>; Ulrich Weigand <
> Ulrich.Weigand at de.ibm.com>
> *Cc:* LLVM Developers <llvm-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] Should rint and nearbyint be always constrained?
>
>
>
> I agree with Ulrich. The default behavior of LLVM IR is to assume that the
> roundToNearest is the current rounding mode everywhere. This corresponds to
> the C standard, which says that the user may only modify the floating point
> environment if fenv access is enabled. In the latest version of the C
> standard, pragmas are added which can change the rounding mode for a
> region, and if these are implemented in clang the constrained versions of
> all FP operations should be used. However, outside of regions where fenv
> access is enabled either by pragma or command line option, we are free to
> assume that the current rounding mode is the default rounding mode.
>
>
>
> So, llvm.rint and llvm.nearbyint (the non-constrained versions) can be
> specifically documented as performing their operation according to
> roundToNearest and clang can use them in the default case for the
> corresponding libm functions, and llvm.experimental.constrained.rint and
> llvm.experimental.constrained.nearbyint can be documented as using the
> current rounding mode.
>
>
>
> The only issue I see is that since we also assume FP operations have no
> side effects by default there is no difference between llvm.rint and
> llvm.nearbyint. I wouldn’t have a problem with dropping llvm.rint
> completely.
>
>
>
> As for the target-specific intrinsics, you are correct that we need a plan
> for that. I have given it some thought, but nothing is currently
> implemented. My suggestion would be that we should set the strictfp
> attribute on these intrinsics and provide the rounding mode and exception
> behavior arguments using an operand bundle. We do still need some way to
> handle the side effects. My suggestion here is to add some new attribute
> that means “no side effects” in the absence of the strictfp attribute and
> something similar to “inaccessibleMemOnly” in the presence of strictfp.
>
>
>
> We could make the new attribute less restrictive than inaccessibleMemOnly
> in that it only really needs to act as a barrier relative to other things
> that are accessing the fp environment. I believe Ulrich suggested this to
> me at the last LLVM Developer Meeting.
>
>
>
> -Andy
>
>
>
> *From:* Serge Pavlov <sepavloff at gmail.com>
> *Sent:* Monday, March 02, 2020 8:10 AM
> *To:* Ulrich Weigand <Ulrich.Weigand at de.ibm.com>
> *Cc:* Kaylor, Andrew <andrew.kaylor at intel.com>; Cameron McInally <
> cameron.mcinally at nyu.edu>; Kevin Neal <kevin.neal at sas.com>; LLVM
> Developers <llvm-dev at lists.llvm.org>
> *Subject:* Re: Should rint and nearbyint be always constrained?
>
>
>
> I'm not sure why this is an issue.  Yes, these two intrinsics depend
> on the current rounding mode according to the C standard, and yes,
> LLVM in default mode assumes that the current rounding mode is the
> default rounding mode.  But the same holds true for many other
> intrinsics and even the arithmetic IR operations like add.
>
>
>
> Any other intrinsic, like `floor`, `round` etc has meaning at default
> rounding mode. But use of `rint` or `nearbyint` in default FP environment
> is strange, `roundeven` can be used instead. We could use more general
> intrinsics in all cases, as the special case of default environment is not
> of practical interest.
>
>
>
> There is another reason for special handling. Set of intrinsics includes
> things like `x86_sse_cvtss2si`. It is unlikely that all of them eventually
> get constrained counterpart. It looks more natural that such intrinsics are
> defined as accessing FP environment and can be optimized if the latter is
> default. These two intrinsics could be a good model for such cases. IIUC,
> splitting entities into constrained or non-constrained is a temporary
> solution, ideally they will merge into one entity. We could do it for some
> intrinsics now.
>
>
>
> Thanks,
> --Serge
>
>
>
>
>
> On Mon, Mar 2, 2020 at 8:58 PM Ulrich Weigand <Ulrich.Weigand at de.ibm.com>
> wrote:
>
> Serge Pavlov <sepavloff at gmail.com> wrote on 02.03.2020 14:38:48:
>
> > This approach has issues when applied to the intrinsics `rint` and
> > `nearbyint`. Value returned by either of these intrinsics depends on
> > current rounding mode. If they are considered as operation in
> > default environment, they would round only to nearest. It is by far
> > not the meaning of the standard C functions that these intrinsics
> represent.
>
> I'm not sure why this is an issue.  Yes, these two intrinsics depend
> on the current rounding mode according to the C standard, and yes,
> LLVM in default mode assumes that the current rounding mode is the
> default rounding mode.  But the same holds true for many other
> intrinsics and even the arithmetic IR operations like add.
>
> If you want to stop clang from making the default rounding mode
> assumption, you need to use the -frounding-math option (or one
> of its equivalents), which will cause clang to emit the corresponding
> constrained intrinsics instead, for those two as well all other
> affected intrinsics.
>
> I don't see why it would make sense to add another special case
> just for those two intrinsics ...
>
>
> Bye,
> Ulrich
>
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